The present invention relates to an autofocus control apparatus and method applied to an optical device such as a microscope.
Conventionally, various types of autofocus detection of autofocus detection apparatuses applied to optical devices such as a microscope have been available.
The first example is the following method disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2-109008. In autofocus control for controlling a photographing optical system to focus on an object image formed on the image pickup screen via an imaging optical system, the focus area corresponding to the object image is divided into a plurality of detection areas. Focus control for the imaging optical system is performed using the focusing information of these areas except for ambiguous focusing information.
The second example is the following method disclosed in Jpn. Pat. Appln. KOKAI Publication No. 57-5016. An image formed by an imaging optical system is scanned using a scan means to obtain a scan signal representing the illuminance distribution of the image. This scan signal is supplied to an illuminance change detector to obtain a signal representing an illuminance change at each small portion in the image. Based on this illuminance change signal, an image visibility signal on the scan surface is obtained. In this case, a differentiating circuit having adjustable operation characteristics is used as the illuminance change detector, and an adjustment circuit for adjusting the operation characteristics of this differentiating circuit is arranged. This adjustment circuit adjusts the operation characteristics and gain of the differential circuit on the basis of the numerical aperture information of the imaging optical system.
According to a method of selecting an area suitable for focus detection from an image passing through the imaging optical system, like the method of the first example, e.g., when a high-magnification objective lens is switched to a low-magnification objective lens in a microscope to observe a specimen, the specimen observation range is narrow in observation by the high-magnification objective lens. For this reason, if the high-magnification objective lens is used, the specimen image may fall outside the field of view or the range of a focus detection sensor depending on the observation position of the specimen, disabling focus detection. When the offset amount from the focal point is large, the specimen image is hard to observe, so an area suitable for focus detection cannot be selected from this image. For this reason, this method is effective only after the lens comes near the focal position.
According to a method of changing the visibility evaluation function of an image depending on the conditions of an imaging optical system to always perform appropriate image evaluation, like the second example, e.g., when this method is applied to a microscope, the frequency distribution of the specimen image to be observed varies depending on each specimen even after the specimen image passes through the same imaging optical system. For this reason, even if the visibility evaluation function of the image is changed, it is not always appropriate depending on the conditions of the imaging optical system.
It is an object of the present invention to provide an autofocus control apparatus and method in which high-speed autofocus detection can be attained with high precision, and stable focus detection can be realized.
It is another object of the present invention to provide a microscope in which high-speed autofocus detection can be attained with high precision, and stable focus detection can be realized.